The crosslinking of polymer chains and the density of the crosslinking can be used to alter the physical properties of a given polymer. For example, crosslinking distinguishes a low Tg liquid polymer from an elastomer, and crosslinking can be used to increase the shear or cohesive strength in materials, such as in pressure sensitive adhesives.
Traditionally, there are two different ways of crosslinking polymer chains. The first method involves adding cure-site monomers during the polymerization. Cure-site monomers are incorporated into the polymer chain during the polymerization process. These cure-sites are then subsequently reacted with curing agents to crosslink polymer chains. The second method involves post-polymerization reactions to crosslink the polymer chains.
In post-polymerization reactions, functional sites on an existing polymer are reacted with a molecule (sometimes known as a curative) to connect at least two polymer chains. Typically, the curative comprises at least two of the same reactive groups capable of reacting with functional sites on the existing polymer. Examples of such compounds include: bisamide crosslinking agents as described in U.S. Pat. No. 6,893,718 (Melancon et al.), for example, 1,1′-(1,3-phenylene dicarbonyl)-bis-(2-methylaziridine), 1,4-bis(ethyleneiminocarbonylamino)benzene, 4,4′-bis(ethyleneiminocarbonylamino)diphenylmethane, and 1,8-bis(ethyleneiminocarbonylamino)octane; and diisocyanate-type crosslinking agent, for example, 1,4-tolylene diisocyanate, and hexamethylene diisocyanate (HDI), sold for example, under the trade designation “DESMODUR N-3200” biuret of HDI from Bayer, Pittsburgh, Pa.), “CORONATE L” and “CORONATE L-55E” from Nippon Shokubai Co., LTD., Tokyo, Japan. In these post-polymerization reactions, one of the reactive groups reacts with one polymer chain, while the other group reacts with another polymer chain to crosslink the polymer. Because these compounds comprise at least two of the same reactive groups, the modification and crosslinking of the polymer chains occur under the same reaction conditions. Thus, using these curatives for the crosslinking of polymers post-polymerization may lead to curable polymeric compositions with short pot lives under certain processing conditions (i.e., the curable polymeric compositions begins to prematurely crosslink).
Recently, it has become known to form a 1,3-cyclo-addition of azides with terminal acetylene (also known as a 3+2 cycloaddition) using a copper catalyst at room temperature in what is known as a “click reaction”. Katritzky, et al., in J. Poly. Sci.: Part A, v. 46, 238-256 (2008), describe the preparation and characterization of end-capped azides and alkynes, wherein the azides were combined in 1,3-dipolar cycloaddition reactions to form triazole linked polymers. These reactions have had limited use in polymeric systems.
In U.S. Prov. application Ser. No. 12/271,222 (Crandall, et al.), a novel cure system for polymers is described comprising azides and non-activated acetylene cure systems. Azido-derivitized and/or acetylene derivitized (meth)acrylate monomers were synthesized and then subsequently used in the polymerization of (meth)acrylate polymers to create crosslinking sites in the polymer chain. These polymers comprising the crosslinking sites were then reacted with the corresponding curing agent at room temperature to crosslink the polymer. For example, a polymer comprising azido cure sites was reacted with a non-activated acetylene curing agent or a polymer comprising non-activated acetylene cure sites.
In Adv. Funct. Mater. 2009, 19, 1-7, the use of these 3+2 cycloaddition reactions to generate a polymeric coating were disclosed. In one embodiment, methyl methacrylate was copolymerized with 2-azidoethyl methacrylate via a free radical polymerization. The polymerization failed, which was thought to be a result of the azides reacting with the propagating radicals, resulting in uncontrolled network formation.